This invention relates to a method and apparatus for producing photovoltaic cells by depositing layers of amorphous semiconductor materials of varying electrical conductivity onto a web of substrate material as the substrate material is continuously advanced through a series of dedicated deposition chambers. The deposition of each layer occurs in a separate glow discharge deposition chamber wherein isolation of preselected reaction gas mixtures which form the amorphous semiconductor layers substantially eliminates cross-contamination between adjacent chambers. The substrate is preferably formed of stainless steel and is adapted to be fed sequentially and continuously through the dedicated chambers to have deposited thereon two or more band gap-adjusted, amorphous, photoresponsive cells such as a plurality of p-i-n-type photovoltaic cells. This invention has its most important application in the mass production of tandem band gap adjusted, amorphous semiconductor, photoresponsive, high efficiency solar cells.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor films, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices substantially equivalent to those produced by their crystalline counterparts. For many years such work with amorphous silicon or germanium films was substantially unproductive because of the presence therein of microvoids and dangling bonds which produce a high density of localized states in the energy gap. Initially, the reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein silane (SiH.sub.4) gas is passed through a reaction tube where the gas is decomposed by a; radio frequency (r.f.) glow discharge and deposited on a substrate at a substrate temperature of about 500-600 degrees K. (227-327 degrees C.). The material so deposited on the substrate is an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, phosphine gas (PH.sub.3), for n-type conduction, or diborane (B.sub.2 H.sub.6) gas, for p-type conduction is premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The material so deposited includes supposedly substitutional phosphorus or boron dopants and is shown to be extrinsic and of n or p conduction type. The hydrogen in the silane was found to combine, at an optimum temperature, with many of the dangling bonds of the silicon during the glow discharge deposition to substantially reduce the density of the localized states in the energy gap, thereby causing the amorphous material to more nearly approximate the corresponding crystalline material.
It is now possible to prepare greatly improved amorphous silicon alloys, that have significantly reduced concentrations of localized states in the energy gaps thereof, while providing high quality electronic properties by glow discharge. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, it is believed that fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
Activated fluorine is believed to readily diffuse into, and bond to, amorphous silicon in a matrix body to substantially decrease the density of localized defect states therein. This is thought to be due to the fact that the small size of the fluorine atoms enables them to be readily introduced into an amorphous silicon matrix. The fluorine therefore bonds to the dangling bonds of the silicon and forms a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than could be formed by hydrogen, or other compensating or altering agents which were previously employed. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen, because of its exceedingly small size, high reactivity, specificity in chemical bonding, and having highest electronegativity.
Compensation may be achieved with fluorine, alone or in combination with hydrogen, upon the addition of such element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages, permitting the elements to form a silicon-hydrogen-fluorine alloy. Thus, alloying amounts of fluorine and hydrogen may, for example, be used in a range of 0.1 to 5 percent or greater. The alloy thus formed has a lower density of defect states in the energy gap than can be achieved by the mere neutralization of dangling bonds and similar defect states. In particular, it appears that use of larger amounts of fluorine participates substantially in effecting a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to the aforementioned characteristics, is believed to (1) be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects, and also (2) influence the bonding of hydrogen by acting to decrease the density of the defect states which hydrogen normally contributes. The ionic role that fluorine plays in such an alloy is an important factor in terms of the nearest neighbor relationships.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell while the short circuit current remains substantially constant.
Many publications on crystalline stacked cells following Jackson have been reported and, recently, several articles dealing with Si-H materials in stacked cells have been published. Marfaing proposed utilizing silane deposited amorphous Si-Ge alloys in stacked cells, but did not report the feasibility of doing so. (Y. Marfaing, Proc. 2nd European) Communities Photovoltaic Solar Energy Conf., Berlin, West Germany, p. 287, (1979).
Hamakawa et al, reported the feasibility of utilizing Si-H in a configuration which will be defined herein as a cascade type multiple cell. The cascade cell is hereinafter referred to as a multiple cell without a separation or insulating layer there between. Each of the cells was made of an Si-H material of the same band gap as in a p-i-n junction configuration. Matching of the short circuit current (J.sub.sc) was attempted by increasing the thickness of the cells in the serial light path. As expected, the overall device Voc increased and was proportional to the number of cells.
In a recent report on increasing the cell efficiency of multiple-junction (stacked) photovoltaic cells of amorphous silicon deposited from silane in the above manner, it was reported that "(g)ermanium has been found to be a deleterious impurity in Si:H, lowering its J.sub.sc exponentially with increasing Ge . . ." From their work, as well as the work of Carlson, Marfaing and Hamakawa, they concluded that alloys of amorphous silicon, germanium and hydrogen "have shown poor photovoltaic properties" and thus new "photovoltaic film cell materials must be found having spectral response at about 1 micron for efficient stacked cell combinations with a Si:H." (J. J. Hanak, B. Faughnan, V. Korsun, and J. P. Pellican, presented at the 14th IEEE Photovoltaic Specialists Conference, San Diego, Calif., Jan. 7-10, 1980).
Due to the beneficial properties attained by the introduction of fluorine, amorphous alloys used to produce cascade type multiple cells may now incorporate fluorine as one possible method of reducing the density of localized states without impairing the electronic properties of the material. Further band gap adjusting element(s), such as germanium and carbon, can be activated and are added in vapor deposition, sputtering or glow discharge processes. The band gap is adjusted as required for specific device applications by introducing the necessary amounts of one or more of the adjusting elements into the deposited alloy cells in at least the photocurrent generation region thereof. Since the band gap adjusting element(s) has been tailored into the cells without adding substantial deleterious states, because of the influence of fluorine, the cell material maintains high electronic qualities and photoconductivity when the adjusting element(s) are added to tailor the device wavelength characteristics for a specific photoresponsive application. The addition of hydrogen, either with fluorine or after deposition, can further enhance the fluorine compensated or altered alloy. The post deposition incorporation of hydrogen is advantageous when it is desired to utilize the higher deposition substrate temperatures allowed by fluorine.
It is of obvious commercial importance to be able to mass produce photovoltaic devices such as solar cells. However, with crystalline cells mass production was limited to batch processing techniques by the inherent time consuming growth requirements of the crystals. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon semiconductor alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386 filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493 filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; and Ser. No. 306,146 filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other deposition chambers to prevent the back diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by employing isolation mechanisms in which an inert gas flowing over the substrate as it passes from one deposition chamber to an adjacent deposition chamber prevents cross contamination. However, none of those prior art isolation mechanisms are employed with apparatus adapted to produce tandem solar cells in a high volume, continuous processing system.
The many objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which follow.